Located
about 28,000 light-years from Earth, this swarm
of stars, known as M80, is one of the densest
globular star clusters in the Milky Way. The cluster
contains hundreds of thousands of stars, all held
together by their mutual gravitational attraction.
In addition to being beautiful, clusters such
as this one tell us much about stellar evolution
and provide a means of measuring the ages of stars.
All the stars in a cluster like this one were
formed at the same time and so are of the same
age. They are among the oldest stars in our galaxy.
Yet they cover a range in terms of mass and size.
The more massive stars in the cluster burn their
nuclear fuel more quickly and evolve into red
giants and ultimately white dwarfs. The less massive
stars have longer lives and many are still burning
their original hydrogen fuel in their central
cores, much like our own Sun does.

Hubble
is able to resolve individual stars in globular
clusters, covering an unprecedented range of mass,
size, and degree of evolution, and to accurately
measure their brightness and colors. By comparing
these accurate measurements to theoretical models
describing how stars evolve, Hubble can measure
the age of the entire cluster. A lot of work remains
to be done. But Hubble's accurate measurements,
coupled with recent improvements in the measurement
of the clusters' distances, has led to revised
estimates of the ages of the oldest stars in our
galaxy  about 13-14 billion years.